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Summary
A conception date calculator works backwards from a known due date or last menstrual period (LMP) to estimate when fertilization most likely occurred. In a standard 28-day cycle, conception typically happens around day 14 — the day of ovulation. This calculator helps answer the common question “when did I conceive?” by reversing the standard pregnancy dating formulas.
Medical disclaimer: This calculator provides estimates for informational purposes only. The actual conception date can vary based on individual cycle length and ovulation timing. Always consult a qualified healthcare provider for medical advice and prenatal care.
How it works
The calculator supports two approaches:
- From a known due date — subtracts 266 days (38 weeks) from the estimated due date to find the likely conception date.
- From LMP — adds 14 days to the LMP, since ovulation (and therefore conception) is assumed to occur around day 14 of a 28-day cycle.
Both methods produce an estimate. The actual fertile window spans several days, so the exact moment of conception cannot be pinpointed from dates alone.
The formulas
Where
Where
Worked examples
Conception date from a due date of October 8, 2025
Start with the due date
= 2025-10-08
Subtract 266 days
= 2025-01-15
Result
Estimated conception date: January 15, 2025
Conception date from LMP of January 1, 2025
Start with the LMP date
= 2025-01-01
Add 14 days for ovulation
= 2025-01-15
Result
Estimated conception date: January 15, 2025
The fertile window
Conception does not happen at a single precise moment. The fertile window typically spans about 6 days:
- Sperm survival: Sperm can survive in the reproductive tract for up to 5 days.
- Egg viability: A released egg is viable for about 12-24 hours after ovulation.
- Peak fertility: The 2-3 days leading up to and including ovulation day are the most fertile.
This means intercourse several days before ovulation can still result in conception, making it impossible to determine the exact conception date from calendar dates alone.
Assumptions & limitations
- Assumes a 28-day cycle with ovulation on day 14. People with longer or shorter cycles will ovulate on different days.
- Conception window, not a point in time — the calculator gives a single date estimate, but conception could have occurred within a range of several days.
- Does not replace medical dating — first-trimester ultrasound measurements provide more accurate pregnancy dating than calendar-based calculations.
- IVF and assisted reproduction — for medically assisted conceptions, the transfer or insemination date is the known conception date and these formulas are unnecessary.